This invention relates generally to vehicle wheel service systems and, more particularly, to a method and system for determining vehicle wheel runout and correcting for mismatched wheel assembly runout.
At least some known vehicles experience vibration at road speeds due to non-uniformity in the vehicle's wheel assemblies. The wheel assembly includes a tire coupled to a rim. The non-uniformity may be due to imperfections or damage to the rim and/or tire. The non-uniformity may include a spring rate variation in the tire. The spring rate variation relates to a tire's stiffness at areas spaced about the circumference of the tire. As the stiff area rotates into contact with the road, the tire reacts differently than at other areas of the tire. This difference produces a force variation at a rotational speed of the tire, leading to a first harmonic vibration. Force variation is measured while the inflated tire is rolled against an instrumented drum.
Force variation can be measured while the inflated tire is rolled against an instrumented drum or roller. The prior art can also remount the tire at an optimal position relative to the rim during a matching procedure to reduce or eliminate force variation as an assembly. Other prior art can measure unloaded runout using contactless measurement and optimally position the tire relative to the rim based on runout instead of force variation. These matching procedures (on systems with or without a load roller) require measurement of rim runout. The measured rim contribution is then subtracted from the measured assembly result, yielding a tire-only contribution to the assembly effect. Rim runout is typically measured with the tire mounted for convenience reasons, but there are problems associated with measuring runout in this manner. Many of today's wheels do not provide a proper surface to measure rim runout with the tire mounted, and removal of the tire to measure the bare rim directly at the bead seats is too labor intensive for many customers. An additional problem is that errors can happen with both contact and non-contact rim runout measurement with the tire mounted because the physical bead seat is not what is being measured. For example, the bead seats on an aluminum wheel could be machined a the last manufacturing step, resulting in a surface which does not correlate to any of the external rim surfaces accessible by runout measuring devices when the tire is mounted. Another situation can exist where the rim width varies along different angular locations of the rim as shown by the two extreme rim lip positions 160 and 160a of FIG. 2. The narrower sections squeeze the beads closer together, altering the geometric positioning of the tire carcass and correspondingly the assembly radial force variation. The prior art measures lateral rim runout but only as an audit to check the rim, but in the event that tire construction details were to be made known to the wheel service system the lateral rim runout could be used to predict the effect on assembly radial force variation.
The present invention provides a way to determine the contributions of the tire and rim individually without measuring rim runout at all, avoiding the aforementioned drawbacks and providing a more accurate result.